Everything is additive, so if there’s 10% RR in one weapon, you’re dual wielding, you have 20% RR. Chances and duration doesn’t stack.
Example:
20% of X over/for 3 seconds
15% of Y over/for 5 seconds
= 15% of X+Y over/for 3 seconds and 20% of X over/for 3 seconds. The effect that lasts longer will be extended, so after 15% X+Y expires, Y will continue for 2 more seconds.